(1) Field of the Invention
The invention relates to a measuring device comprising a movable measuring probe and sensors coupled to the measuring probe for providing position data of the measuring probe.
(2) Description of Related Art
A measuring device of this kind is known from U.S. patent application Ser. No. 4,703,443, and can be used for measuring the shape or contour of a two-dimensional or three-dimensional object, such as machine components or the like that are placed on a measuring table.
This prior art measuring device comprises an arm that is rotatably mounted on the measuring table, the length of which arm can be varied. The arm consists of a number of separate, pivotally interconnected elements. Disposed on the free end of the arm is a measuring probe, which is likewise pivotally connected thereto.
Angular displacement sensors, such as potentiometers, for measuring the angle between elements disposed adjacently to each other are present at all pivot points of the arm for the purpose of determining the effective length of the arm, that is, from the pivot point to the point where the measuring probe is in contact with the object to be measured. On the basis of the lengths of the individual elements, which are known per se, and the measured angle between said elements it is then possible to determine the effective length of the arm by means of a simple mathematical computation. Subsequently the two-dimensional or three-dimensional contour of an object to be measured can be determined from the length and the angular displacement of the arm by means of well-known mathematical formulas based on a Cartesian, spherical or cylindrical coordinate system.
The presence of a relatively large number of sensors at the pivot points of the arm for determining the length thereof makes the arm relatively vulnerable and liable to malfunction, in particular when potentiometers are used. Furthermore it will be apparent that the maximum length of the arm is inevitably limited for constructional reasons, so that this prior art device is only suitable for measuring relatively small objects, which can be placed on a measuring table.
Consequently it is an object of the invention to provide an improved measuring device, which has been designed for measuring small objects to be placed on a measuring table, as well as relatively large objects disposed in a room.
According to the invention this object has been accomplished in that the measuring probe is coupled, via a cord or a wire, to a first sensor for measuring the length or change in length of the cord or the wire, and to a second sensor for measuring an angle or angular displacement of the cord or the wire.
Instead of using a pivotable arm the device according to the invention uses a cord or a wire, so that a single sensor will suffice for determining the length or change in length of the cord or the wire, and a measuring device that is much less liable to malfunction can be provided. By using a cord or a wire also the inherent limitation as regards the length of the prior art pivotable arm has been overcome, as a result of which it is also possible to measure relatively large objects that are disposed in a room. By way of illustration, in a practical embodiment of the measuring device according to the invention, measurement is carried out with a cord or wire having a length of 6 meters or more.
In an embodiment of the device according to the invention, in order to enable accurate determination of the angle or angular displacement of the cord caused by a change in position of the measuring probe, the second sensor is coupled to an elongate, rotatably supported arm, in the longitudinal direction of which the cord or the wire engages the arm.
In a preferred embodiment of the measuring device according to the invention, the arm is coupled to the second sensor at a first end, and is provided at a second free end with an opening precisely adapted to the thickness of the cord or the wire, through which opening the cord or the wire can be moved. The clearance between the wire and the opening must be minimal in order to keep the arm in line with the cord or the wire as accurately as possible.
In an embodiment of the measuring device according to the invention the arm is supported in such a manner that it is rotatable in an imaginary plane, in particular for measuring two-dimensional objects. On the basis of the measured angle or angular displacement of the arm and the measured length or change in length of the cord or the wire, the contour of an object scanned by means of the measuring probe can be accurately determined by means of mathematic computations based on a polar coordinate system.
Since the accuracy of such a measurement is determined, among other things, by the accurate rotation of the arm in said imaginary plane, the arm of another embodiment of the measuring device according to the invention is rotatably supported at a point some distance away from its first end, in such a manner that the first end of the arm extending beyond the point of support is suitably shaped for balancing the arm, or that said end can be provided with adjusting means for balancing the arm in said imaginary plane with the desired accuracy.
In yet another embodiment of the measuring device according to the invention the arm is spatially rotatably supported, for example by means of a ball joint mounted on the first end of the arm for swingably supporting the arm. An arm which is spatially rotatable or swingable in this manner is suitable for measuring contours of three-dimensional objects, for example based on the well-known spherical coordinate system.
The speed at which the measurement can be carried out depends, among other things, on the speed at which the arm is capable of following the changes in position of the cord or the wire.
In the preferred embodiment of the invention the arm is supported in a precision bearing having the smallest possible starting moment, that is, the moment that is required for causing the bearing to rotate from standstill.
In yet another embodiment of the invention the arm is made of a material having a low specific weight, such as aluminium or a plastic, wherein the arm is furthermore designed to comprise as little material as possible whilst retaining sufficient mechanical strength, however.
For an accurate measurement of the length or change in length of the cord or the wire caused by a change in position of the measuring probe, the cord or the wire needs to be sufficiently taut when the position of the probe is being determined.
According to an embodiment of the invention, again in order to enhance the speed of the measuring operation, the first sensor is coupled to a tensioning and roll-up mechanism for keeping the cord or wire tensioned under the influence of spring tension and for automatically rolling up said cord or wire. Thus it is achieved that the cord or the wire is sufficiently tensioned for carrying out the measurement at all times so as to be able to accurately determine the length or change in length of the cord or the wire.
In a preferred embodiment of the measuring device according to the invention the tensioning and roll-up mechanism comprises a rotatably supported reel, whose outer surface is provided with a spiral groove that has a depth adapted to the diameter of the cord or the wire, and movably supported guide wheels for guiding the cord or the wire in such a manner that it will follow said spiral groove of the reel.
This embodiment of the tensioning and roll-up mechanism prevents the cord or the wire from heaping up upon being wound onto the reel, which would introduce an error into the measurement. After all, the length of the cord or the wire is determined by the diameter of the reel, wherein the heaping up of layers of cord or the wire in fact corresponds to unknown changes in the diameter of the reel. Furthermore, the cord or the wire is prevented from being flattened upon being wound up as a result of several layers of cord or the wire being wound one on top of another. Said flattening of the cord or the wire in turn results in an unknown variation in the determination of the changes in length and consequently in an unknown measuring error.
In the preferred embodiment of the invention the measuring probe is elongate in shape, comprising a grip for taking hold of the measuring probe and a pin-shaped end that is rotatably supported with respect to said grip, to which said cord or the wire is attached.
By attaching the cord or the wire to the rotatably supported pin-shaped end it is achieved that the cord or the wire will extend in the radial direction of the measuring probe at all times during the positioning of the measuring probe. Consequently, no errors are introduced into the linear measurement, because the wire is not in line with the centre of the measuring probe and/or with the pin-shaped end thereof.
In particular for measuring relatively large objects, wherein the cord or the wire can have a length of 6 meters or more, a material which is as low-stretch as possible should be selected, because stretch of the cord makes the linear measurement unreliable. It has become apparent that paraleine cord or paraleine wire is sufficiently stretch-proof for the purpose of the invention. Of course also the spring tension of the tensioning and roll-up mechanism must be selected in such a manner that it will not cause undesirable stretch of the cord or the wire.
Sensors that are suitable for the purpose of the invention are known per se in practice. In a preferred embodiment of the invention the sensors are in the form of pulse generators, wherein the number of pulses delivered during use is proportional to a change in length or angular displacement of the cord or the wire or of the arm coupled thereto.
The sensors and the rotatable arm of the measuring device according to the invention can be accommodated in a compact, portable housing, from where the measuring probe and the cord or the wire can be displaced. The housing can be placed on a measuring table or at any other point in a room for measuring a respective object.
In yet another embodiment of the invention, in order to facilitate using the measuring device with existing computer apparatus, such as a desk computer or a portable computer (laptop), the measuring device is characterized by a processing device connected to the sensors for processing measuring signals delivered by the sensors into position data of the measuring probe and making said data available on an interface. The interface is preferably a standardized interface for use with computer peripherals, such as the RS 232, which is known per se, or the like.
In the most complete embodiment the measuring device comprises a further processing device connected to the interface, such as a desk computer or a portable computer, which is provided with suitable software for processing the position data for driving a device for graphically representing the obtained position data, and/or software for processing said position data for driving a machining apparatus for directly and automatically producing an object corresponding to the position data. Graphical devices or plotters and machining apparatus that are suitable for this purpose are known per se in the industry.
Consequently, the invention also provides apparatus for graphically representing measured position data, characterized by a measuring device as described above, which is connected to said processing apparatus.
The invention furthermore relates to machining apparatus for automatically producing objects on the basis of predetermined measuring data, characterized by a measuring device as described above, which is connected to said processing apparatus.
The measuring device according to the invention can be used for measuring an object by positioning the measuring probe at one or more points on the circumference of the object in question, wherein the processing device automatically produces a possible contour of the object corresponding with said points, or by moving the measuring probe continuously along the circumference of the object to be measured.
Measuring at discrete points can be carried out, for example, when the object to be measured has a shape which is known per se, for example a polygonal shape. By positioning the measuring probe at the corner points, such an object can be measured quickly. In the case of complex shapes or of precision measurement, it is preferred to move the measuring probe along the circumference of the object to be measured.
In another embodiment of the measuring device according to the invention a measuring-probe is provided which includes a pivotable ball- or sphere-shaped top, in the centre of which the cord or the wire is attached. This measuring probe is in particular suitable for tipping points on an object to be measured, for example in the case of a frame or the like to be measured, independently of the position of the probe.
For calibration purposes, the measuring probe is positioned at a reference point prior to measuring an object, wherein the measurement is calibrated in relation to said reference point.
The invention furthermore relates to a method for measuring an object by means of a measuring device as described above, wherein the measuring data obtained by moving a measuring probe, which data is representative of linear changes and angle changes of the cord or the wire, is subjected to correction operations, among which radius correction of the reel on which the cord or the wire is wound, compensation of vibrations in the measuring arm coupled to the cord or the wire, measuring point and line filtering and offset correction in relation to the dimensions of a measuring pin or measuring top of a measuring probe.
The measuring device according to the invention makes it possible to measure objects very accurately, in the order of tenths of a millimeter, and quickly, and the measuring data can be fed directly to a graphic apparatus or drawing apparatus for providing a graphic representation of the measured object and/or to machining apparatus for directly and automatically producing the object, for example in accordance with the DIN ISO code.
The measuring device according to the invention will be explained in more detail hereafter by means of exemplary embodiments.